Firefox 4 has released a video that shows ants running around:

http://www.mozilla.com/en-US/firefox/4.0b7/ants/

So I ran it on my MacBook 2.4GHz Core 2 Duo on each of my browsers:

• Firefox 3.6.12 = 40 fps - drops to 12 over time
• Firefox 4b7 = 80 fps - stays fairly constant > 70 fps
• Safari 5 = 90 fps - stays fairly constant > 85 fps
• Chrome 7 = 110 fps - moves between 100 and 140 fps

Now I don't know the validity of this test as a basis of real speed comparison, but if one was to take those numbers as is, it shows that Firefox 4 is the slowest of the new browsers.

I am making no judgement at this stage, just printing the numbers I found.